1. Field of Invention
The technology described relates to integrators and methods of operation of the same.
2. Discussion of Related Art
Integrators are commonly used in various types of circuits. Timing circuits, charge measurement circuits, and signal processing circuits all may implement one or more integrators. A specific example of a circuit which may implement an integrator is a Sigma Delta Modulator circuit. The Sigma Delta Modulator may include a filter, which can be formed using one or more integrators in an appropriate configuration.
Integrators can take a variety of forms depending on the environment in which they are used and the desired operating characteristics. When selecting or designing an integrator for a particular application, a circuit designer may consider factors such as power consumption, linearity, size, ease of processing, and compatibility with surrounding circuitry. Thus, while a given integrator design may be beneficial in some settings, it may have significant drawbacks in other settings.
One example of a known integrator design is the fully-differential integrator. FIGS. 1A and 1B illustrate an example of an inverting fully-differential active RC integrator 100, shown in schematic and a more detailed representation, respectively. As shown in FIG. 1A, the fully-differential integrator 100 is configured to receive a differential input signal having positive and negative components Vin+ and Vin−, and output a differential output signal having positive and negative components Vout+ and Vout−. The fully-differential integrator 100 includes an amplifier circuit 102 which has two input terminals, one configured to receive each of the components of the differential input signal Vin. The two components of the differential input signal Vin may be provided to the amplifier 102 via respective resistors, R1 and R2. The amplifier provides the differential output signal Vout from two output terminals. Feedback paths are included in the circuit design, and include respective capacitances, illustrated as capacitors, C1 and C2. The fully-differential integrator 100 is referred to as an active RC integrator because of the presence of the resistors coupled with the capacitors, and the use of amplifier 102.
FIG. 1B shows the fully-differential active RC integrator of FIG. 1A in greater detail, and in particular expands on the detail of the amplifier 102 from FIG. 1A. As shown in FIG. 1B, the amplifier 102 can be viewed as having two substantially identical branches coupled together at a tail current source I3. A first branch of the amplifier 102 includes current source I1 coupled to NMOS transistor 104a. The current source I1 is coupled between a supply voltage level Vdd1 and the drain of NMOS transistor 104a. The drain of NMOS transistor 104a is also coupled to capacitor C2, and is the point of the circuit from which the output Vout− is taken.
Similarly, a second branch of the amplifier includes current source I2 coupled to NMOS transistor 104b. The current source I2 is coupled between a supply voltage level Vdd2, which may be the same as Vdd1, and a drain of NMOS transistor 104b. The drain of NMOS transistor 104b is also coupled to capacitor C1, and is the point of the circuit from which the output Vout+ is taken.
As shown, the first and second branches of the amplifier join at a tail current source I3, which could be a transistor. In particular, the source terminals of NMOS transistors 104a and 104b are coupled to tail current source I3. The tail current source I3 is also coupled to ground. The combination of current sources I1-I3 and the two NMOS transistors 104a and 104b constitute an operational transconductance amplifier (OTA), outlined by box 102.
Another example of a known integrator design is illustrated in FIG. 2. The pseudo-differential active RC integrator 200 is similar to the fully-differential active RC integrator 100 of FIG. 1B, except that the tail current source I3 in FIG. 1B is removed. The source terminals of NMOS transistors 204a and 204b are therefore coupled directly to ground. Since the input common-mode may not coincide with the gate to source voltage, Vgs, of transistors 204a and 204b, the pseudo-differential active RC integrator also includes current sources I4 and I5 to provide level shifting. Current sources I4 and I5 can be implemented as transistors. The current sources I4 and I5 are coupled between respective virtual ground nodes, 211a and 211b (corresponding to the gate terminals of transistors 204a and 204b) and ground.